Exhibit 5.1

DLA Piper LLP (US)

The Marbury Building

6225 Smith Avenue

Baltimore, Maryland 21209-3600

www.dlapiper.com

T  410.580.3000

F  410.580.3001

March 20, 2013

Anworth Mortgage Asset Corporation

1299 Ocean Avenue, Second Floor

Santa Monica, California 90401

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

We serve as special Maryland counsel to Anworth Mortgage Asset Corporation, a Maryland corporation (the “Company”), and have been requested to render this opinion in connection with the Registration Statement on Form S-3 of the Company (the “Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) on March 20, 2013, including the preliminary prospectus included therein at the time the Registration Statement becomes effective (the “Prospectus”), under the Securities Act of 1933, as amended (the “Securities Act”), for registration by the Company of $545,055,196 aggregate initial offering price of Securities (as defined below).

As used herein, the term “Securities” includes (i) shares of preferred stock, par value $0.01 per share, of the Company in one or more classes or series as designated by the Company (the “Preferred Stock”), (ii) shares of common stock, par value $0.01 per share, of the Company (the “Common Stock”) and (iii) warrants to purchase Preferred Stock or Common Stock of the Company, as designated by the Company at the time of the offering (collectively, the “Warrants”). The Registration Statement provides that the Securities may be offered separately or together, in separate series, in amounts, at prices and on terms to be set forth in one or more supplements to the Prospectus (each, a “Prospectus Supplement”) and any related free writing prospectus (each, a “Free Writing Prospectus”).

In rendering the opinion expressed herein, we have reviewed originals or copies, certified or otherwise identified to our satisfaction, of the following documents (collectively, the “Documents”):

(a) the Registration Statement, including the Prospectus;

(b) the charter of the Company, as in effect on the date hereof (the “Charter”), certified by the State Department of Assessments and Taxation of the State of Maryland (the “SDAT”);

(c) the by-laws of the Company, as in effect on the date hereof (in the form attached to the Certificate (as defined below)) (the “By-Laws”);

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March 20, 2013

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(d) certified resolutions of the Board of Directors of the Company (the “Board”) on March 18, 2013 relating to the authorization of the filing of the Registration Statement (in the form attached to the Certificate);

(e) a good standing certificate for the Company, dated as of March 18, 2013, issued by the SDAT; and

(f) a certificate executed by the Secretary of the Company (the “Certificate”), dated as of the date hereof, as to certain factual matters.

In examining the Documents, and in rendering the opinion set forth below, we have assumed, without independent investigation, the following: (a) each of the parties to the Documents (other than the Company) has duly and validly authorized, executed and delivered each of the Documents and each instrument, agreement and other document executed in connection with the Documents to which such party (other than the Company) is a signatory and each such party’s (other than the Company’s) obligations set forth in the Documents, are its legal, valid and binding obligations, enforceable in accordance with their respective terms; (b) each person executing any such instrument, agreement or other document on behalf of any such party (other than the Company) is duly authorized to do so; (c) each natural person executing any such instrument, agreement or other document is legally competent to do so; (d) the Documents accurately describe and contain the mutual understandings of the parties, there are no oral or written modifications of or amendments or supplements to the Documents and there has been no waiver of any of the provisions of the Documents by actions or conduct of the parties or otherwise; and (e) all documents submitted to us as originals are authentic, all documents submitted to us as certified or photostatic copies or telecopies or portable document file (“.PDF”) copies conform to the original documents (and the authenticity of the originals of such copies), all signatures on all documents submitted to us for examination (and including signatures on photocopies, telecopies and .PDF copies) are genuine, and all public records reviewed are accurate and complete. As to all factual matters relevant to the opinion set forth below, we have relied on the Certificate as to the factual matters set forth therein, which we assume to be accurate and complete, and on the written statements and representations of public officials and others and our review of the Documents.

We further assume that:

(a) The issuance, sale, amount and terms of the Securities to be offered from time to time by the Company will be authorized and determined by proper action of the Board (or where permitted, a committee of the Board) of the Company (each, a “Board Action”) in accordance with the Company’s Charter, By-Laws and applicable law, in each case so as not to result in a default under or breach of any instrument, document or agreement binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental or regulatory body having jurisdiction over the Company.

(b) The Company will not issue any Securities in excess of the number or amount authorized by a Board Action.

(c) The prior issuance of all of the issued and outstanding shares of Common Stock of the Company has not violated and the issuance of the Securities and shares of Common Stock and shares of Preferred Stock issuable upon exercise of the Warrants will not violate the ownership limitations set forth in the Charter.

(d) Prior to the issuance of any shares of Preferred Stock (including Preferred Stock issuable upon exercise of Warrants), or Common Stock (including Common Stock issuable upon

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exercise of Warrants), there will exist, under the Company’s Charter, the requisite number of authorized but unissued shares of Preferred Stock (and securities of any class into which any of the Preferred Stock may be convertible), or Common Stock, as the case may be, and all actions necessary to the creation of any such Preferred Stock (and securities of any class into which any Preferred Stock may be convertible), whether by articles supplementary or by classification or reclassification of existing capital stock and the filing of articles supplementary with the SDAT, will have been taken.

(e) For shares of Preferred Stock or Common Stock represented by certificates (“Certificates”), appropriate Certificates representing shares of Preferred Stock or Common Stock will be executed and delivered upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter, By-Laws and applicable law. For shares of Preferred Stock or Common Stock not represented by certificates, the applicable Board Action shall have been taken and, upon request of a stockholder of the Company, appropriate written statements (“Written Statements”) will be prepared and delivered to such stockholder upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter, By-Laws and applicable law.

(f) Any Warrants will be issued under a valid and legally binding warrant agreement (a “Warrant Agreement”) that conforms to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus and will comply with the Company’s Charter, By-Laws and applicable law.

(g) To the extent that the obligations of the Company under any Warrant Agreement may be dependent upon such matters, the financial institution to be identified in such Warrant Agreement as warrant agent (the “Warrant Agent”) will be duly organized, validly existing and in good standing under the laws of its jurisdiction of organization; the Warrant Agent will be duly qualified to engage in the activities contemplated by such Warrant Agreement; such Warrant Agreement will have been duly authorized, executed and delivered by the Warrant Agent and will constitute the legally valid and binding obligation of the Warrant Agent enforceable against the Warrant Agent in accordance with its terms; the Warrant Agent will be in compliance, generally, with respect to acting as Warrant Agent under such Warrant Agreement and with applicable laws and regulations; and the Warrant Agent will have the requisite organizational and legal power and authority to perform its obligations under such Warrant Agreement.

(h) The underwriting or other agreements for offerings of the Securities (each, an “Underwriting Agreement,” and, collectively, the “Underwriting Agreements”) will be valid and legally binding contracts that conform to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus.

(i) The Registration Statement will become effective by the Commission and will remain effective under the Securities Act.

Based upon the foregoing, and subject to the assumptions, limitations and qualifications stated herein, it is our opinion that:

(1) Upon an issuance of Certificates or Written Statements, if any, as the case may be, representing shares of Common Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Company’s Charter, By-Laws

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and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, for shares of Common Stock pursuant to the exercise of one or more Warrants or the conversion of one or more series of Securities convertible into Common Stock, such shares of Common Stock will be duly authorized, validly issued, fully paid and non-assessable.

(2) Upon issuance and delivery of Certificates or Written Statements, if any, as the case may be, representing shares of such class or series of Preferred Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Company’s Charter, By-Laws and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, representing shares of such class or series of Preferred Stock pursuant to the exercise of one or more Warrants or the conversion of one or more series of Securities convertible into Preferred Stock, such shares of such class or series of Preferred Stock will be duly authorized, validly issued, fully paid and non-assessable.

(3) Upon execution, issuance and delivery of the Warrants against payment therefor in accordance with the terms and provisions of the applicable Board Action, the Company’s Charter, By-Laws and applicable law, the Warrant Agreement, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, the Warrants will be duly authorized and constitute valid and legally binding obligations of the Company.

In addition to the qualifications set forth above, the foregoing opinion is further qualified as follows:

(a) The foregoing opinion is rendered as of the date hereof. We assume no obligation to update such opinion to reflect any facts or circumstances that may hereafter come to our attention or changes in the law which may hereafter occur.

(b) We do not express any opinion herein concerning any law other than the Maryland General Corporation Law (including the statutory provisions, all applicable provisions of the Maryland Constitution and the reported judicial decisions interpreting the foregoing). The opinion expressed herein is subject to the effect of judicial decisions which may permit the introduction of parol evidence to modify the terms or the interpretation of agreements.

(c) We express no opinion as to compliance with the securities (or “blue sky”), broker licensing, real estate syndication or mortgage lending laws of any jurisdiction.

(d) The opinion stated herein relating to the validity and binding nature of obligations of the Company is subject to (i) the effect of any applicable bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting creditors’ rights generally and (ii) the effect of general principles of equity (regardless of whether considered in a proceeding in equity or at law).

(e) This opinion is limited to the matters set forth herein, and no other opinion should be inferred beyond the matters expressly stated.

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We hereby consent to the filing of this opinion with the Commission as Exhibit 5.1 to the Registration Statement and to the reference to our firm under the heading “Legal Matters” in the preliminary prospectus forming part of the Registration Statement. Greenberg Traurig, LLP is authorized to rely on this opinion as if the opinion were addressed to it solely for the purpose of rendering its opinion to be filed as Exhibit 8.1 to the Registration Statement. In giving our consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ DLA PIPER LLP (US)